Mangekyou Sharingan

[XanBcoo]

For reference, according to AonE's translation of episode 131, Itachi tells Sasuke: "If you awaken it, there will be three people, including me, who have ever used the Mangekyou Sharingan."

That's all the info we have. We know it is not Obito because as you said, he died just after gaining his sharingan. Kakashi might have developed Mangekyou shortly after the Kakashi Gaiden and just has not used it (or has been unable to use it effectively) until the chapter he uses it in (haven't read it, just going off of what I've heard). It's also possible that he only recently developed the MS during the 3 year gap, and the 3rd user Itachi referred to was either someone who lived long ago, or someone who has yet to have been revealed to us.

It's kind of a moot point.

[Divinity]

To get Mangekyou, you have to kill your best friend. So this is where it gets confusing. Time will only tell.

EDIT: OMG OMG OMG I KNOW!! Kakashi killed Gai!!

[XanBcoo]

There's a chance that Kakashi "killed" his best friend (Obito) because he really beleived Obito's death to be his own fault.

[The Heretic Azazel]

Oooh.. the plot thickens.

[Carnage]

But then couldnt that just make it so all the character has to do is believe he killed his friend to get the M. sharingan? Then sasuke could just imagine that he killed Naruto to get the M. sharingan.